Photodiode having voltage tunable spectral response

ABSTRACT

A photodetector ( 10 ) includes a substrate ( 12 ) having a surface; a first layer ( 14 ) of semiconductor material that is disposed above the surface, the first layer containing a first dopant at a first concentration for having a first type of electrical conductivity; and a second layer ( 16 ) of semiconductor material overlying the first layer. The second layer contains a second dopant at a second concentration for having a second type of electrical conductivity and forms a first p-n junction ( 15 ) with the first layer. The second layer is compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap in a direction away from the p-n junction. The compositional grading can be done in a substantially linear fashion, or in a substantially non-linear fashion, e.g., in a stepped manner. Preferably the first dopant concentration is at least an order of magnitude greater than the second concentration, and more preferably is at least two orders of magnitude greater. When the first p-n junction is reverse biased, a depletion region ( 17 ) exists substantially only within the second layer, and varying the magnitude of the reverse bias shifts the wavelength at which a maximum spectral sensitivity or responsiveness is obtained. At least one electrical contact is provided for coupling the second layer to a source ( 32 ) of variable bias voltage for reverse biasing the p-n junction. As the magnitude of the bias voltage is changed a wavelength of electromagnetic radiation to which the photodetector is responsive is thus changed. An alternating current signal can be superimposed on the reverse DC bias voltage and a synchronous detection technique used to detect photons corresponding to a certain bandgap energy.

TECHNICAL FIELD:

These teachings relate generally to detectors of electromagneticradiation and, more specifically, relate to photodiode detectors thatare responsive to electromagnetic radiation in more than one spectralband. Even more specifically, these teachings relate to detectors ofelectromagnetic radiation that have an electrically tunable response tolight of different wavelengths.

BACKGROUND:

Electromagnetic radiation detectors that are responsive to light in morethan one wavelength band, also referred to as multi-spectral ormulti-color detectors, provide a number of advantages in modern imagingsystems. In general, the light that is detected may be visible light orlight that is not visible to the human eye (e.g., infrared (IR)radiation).

Early efforts to detect IR radiation within more than one spectral bandhave relied on the use of multiple detector arrays, each having adifferent spectral filter. Multiple detector arrays with differentspectral responses have also been used. The use of a continuouslyvariable wedge filter in conjunction with a detector array is also knownin the art, as is the use of a mechanical spectral filter selector. Forreasons related at least to increased cost, complexity and weight, theseconventional approaches to multi-spectral imaging are disadvantageousfor many applications.

It was thus realized that the detection of IR radiation in two or morespectral bands with a single integrated detector device was a verydesirable alternative to the conventional approaches. Representativeexamples of such detectors can be found in the following commonlyassigned U.S. patents.

In U.S. Pat. No.: 5,113,076 by Eric Schulte, “Two terminal multi-bandinfrared radiation detector”, there is described a radiation detectorthat includes a first heterojunction and a second heterojunction thatare electrically coupled together in series between a first electricalcontact and a second electrical contact. The detector contains at leasta three regions or layers, including a first layer having a first typeof electrical conductivity, a second layer having a second type ofelectrical conductivity and a third layer having the first type ofelectrical conductivity. The first and second heterojunctions arecoupled in series and function electrically as two back-to-back diodes.During use the detector is coupled to a switchable bias source thatincludes a source of positive bias (+Vb) and a source of negative bias(−Vb). With +Vb applied across the detector the first heterojunction isin far forward bias and functions as a low resistance conductor, therebycontributing no significant amount of photocurrent to the circuit. Thesecond heterojunction is in a reverse bias condition and modulates thecircuit current in proportion to the photon flux of an associatedspectral region or color. Conversely, with −Vb applied across thedetector the second heterojunction is in forward bias and contributes nosignificant photocurrent to the circuit while the first heterojunctionis reversed biased and produces a current modulation proportional to theincident flux, where the flux is associated with a different spectralregion.

In U.S. Pat. No.: 5,731,621 to Kenneth Kosai, “Three band and four bandmultispectral structures having two simultaneous signal outputs”, thereis described a solid state array that has a plurality of radiationdetector unit cells, wherein each unit cell includes a bias-selectabletwo color photodetector in combination with either a secondbias-selectable two color detector or a single photodetector. Each unitcell is thus capable of simultaneously outputting charge carriersresulting from the absorption of electromagnetic radiation within twospectral bands that are selected from one of four spectral bands orthree spectral bands.

In U.S. Pat. No.: 5,751,005 by Richard Wyles et al., “Low-crosstalkcolumn differencing circuit architecture for integrated two-color focalplane arrays”, there is described an integrated two-color staring focalplane array having rows and columns of photodetector unit cells, each ofwhich is capable of simultaneously integrating photocurrents resultingfrom the detection of two spectral bands. A readout circuit performs asubtraction function, and includes a differential charge-sensingamplifier in a one-per-column arrangement. The amplifier works incooperation with circuitry located in each unit cell. The subtractionfunction is employed to create a separate Band1 signal from a Band2 and(Band1+Band2) signals generated by each simultaneous two-color detector.The circuit offers low spectral crosstalk between the two spectralbands.

Also by example, in U.S. Pat. No.: 5,959,339 by Chapman et al.,“Simultaneous two-wavelength p-n-p-n infrared detector” there isdisclosed an array that contains a plurality of radiation detectors.Each radiation detector includes a first photoresponsive diode (D1)having an anode and a cathode that is coupled to an anode of a secondphotoresponsive diode (D2). The first photoresponsive diode responds toelectromagnetic radiation within a first band of wavelengths and thesecond photoresponsive diode responds to electromagnetic radiationwithin a second band of wavelengths. Each radiation detector furtherincludes a first electrical contact that is conductively coupled to theanode of the first photoresponsive diode; a second electrical contactthat is conductively coupled to the cathode of the first photoresponsivediode and to the anode of the second photoresponsive diode; and a thirdelectrical contact that is conductively coupled to a cathode of eachsecond photoresponsive diode of the array. The electrical contacts arecoupled during operation to respective bias potentials. The firstelectrical contact conducts a first electrical current induced byelectromagnetic radiation within the first predetermined band ofwavelengths, and the second electrical contact conducts a secondelectrical current induced by electromagnetic radiation within thesecond predetermined band of wavelengths, less an electrical currentinduced by electromagnetic radiation within the first predetermined bandof wavelengths.

The disclosures of these various commonly assigned U.S. patents areincorporated by reference herein in so far as there is no conflict withthe teachings of this invention.

Also of interest to the teachings of this invention is a p-i-i-n(p-type, intrinsic, intrinsic, n-type) detector that is described byBrüggermann et al., “The operational principle of a new amorphoussilicon based p-i-i-n color detector”, J. Appl. Phys. 81(11), 1 Jun.1997, 7666-7672. The device is constructed using two large band gapfront layers of doped and intrinsic hydrogenated amorphous siliconcarbide (a-SiC:H), followed by an intrinsic and a doped a-Si:H layer.These authors report that by band gap engineering an experimental redresponse is maximized at a large reverse bias voltage, whereas the greenresponse has its maximum at low reverse bias voltage. The potentialprofile of the p-i-i-n structure is said to be of crucial importance tothe color detection mechanism. At larger wavelengths the large potentialdrop across the two highly defective front layers assists recombinationin the back part of the device, which leads to the drop in the redresponse at low reverse voltage. For the voltage-dependent shift inspectral sensitivity it is said to be important that photogeneratedcarriers, under green bias illumination, are lost by recombination inthe front part of the device.

Also of interest is an n-i-p-i-i-n detector of a type described by H.Stiebig et al., “Transient Behavior of Optimized nipiin Three-ColorDetectors”, IEEE Transactions on Electron Devices, Vol. 45, No. 7, Jul.1998, 1438-1444. These authors report the detection of the fundamentalcomponents of visible light (blue, green, red) with a multi-spectraltwo-terminal photodiode that is based on amorphous silicon. Thepreferential carrier collection region of the two-terminal device shiftsupon a change of the applied bias voltage, which leads to a colorsensitivity. Structures with controlled bandgap and mobility-lifetimeproduct exhibit a dynamic behavior above 96 dB. Three linearlyindependent spectral response curves can be extracted to generate a RGB(red-green-blue)-signal. Bias voltage switching experiments underdifferent monochromatic illumination conditions were carried out toinvestigate the time-dependent behavior.

SUMMARY OF THE PREFERRED EMBODIMENTS

The foregoing and other problems are overcome, and other advantages arerealized, in accordance with the presently preferred embodiments ofthese teachings.

A photodetector in accordance with the teachings of this inventionincludes a substrate having a surface; a first layer of semiconductormaterial that is disposed above the surface, the first layer containinga first dopant at a first concentration for having a first type ofelectrical conductivity; and a second layer of semiconductor materialoverlying the first layer. The second layer contains a second dopant ata second concentration for having a second type of electricalconductivity and forms a first p-n junction with the first layer. Thesecond layer is compositionally graded through at least a portion of athickness thereof from wider bandgap semiconductor material to narrowerbandgap in a direction away from the p-n junction. The compositionalgrading can be done in a substantially linear fashion, or in asubstantially non-linear fashion, e.g., in a stepped manner. Preferablythe first dopant concentration is at least an order of magnitude greaterthan the second concentration, and more preferably is at least twoorders of magnitude greater. When the first p-n junction is reversebiased, a depletion region exists substantially only within the secondlayer, and varying the magnitude of the bias shifts the wavelength atwhich a maximum spectral sensitivity or responsiveness is obtained. Atleast one electrical contact is provided for coupling the second layerto a source of variable bias voltage for reverse biasing the p-njunction. As the magnitude of the bias voltage is changed a wavelengthof electromagnetic radiation to which the photodetector is responsive ischanged.

As examples, the semiconductor material can be selected from a GroupII-VI material or from a Group III-V material. The first type ofelectrical conductivity can be p-type, and the second type of electricalconductivity can be n-type, or the first type of electrical conductivitycan be n-type, and the second type of electrical conductivity can bep-type.

The photodetector can further include a third layer of semiconductormaterial that is disposed above the second layer, the third layercontaining a third dopant at a third concentration for having the firsttype of electrical conductivity and a fourth layer of semiconductormaterial overlying the third layer. The fourth layer contains a fourthdopant at a fourth concentration for having the second type ofelectrical conductivity and forming a second p-n junction with the thirdlayer, the fourth layer being compositionally graded through at least aportion of a thickness thereof from wider bandgap semiconductor materialto narrower bandgap semiconductor material in a direction away from thesecond p-n junction. The third concentration is at least an order ofmagnitude greater than the fourth concentration, and when the second p-njunction is reverse biased a depletion region exists substantially onlywithin the fourth layer.

Also disclosed is an array of IR radiation responsive photodetectorswherein each photodetector includes a photodiode having a p-n junction.A wavelength at which a maximum spectral response of the photodiodeoccurs is determined at least in part by a magnitude of a reverse biasvoltage applied to the p-n junction. Each of the photodiodes includes alayer of semiconductor material that is compositionally graded fromwider bandgap material towards narrower bandgap material in a directionaway from the p-n junction. The compositionally graded layer confinessubstantially all of a depletion region of the photodiode.

A method is also disclosed for operating an array of electromagneticradiation responsive photodetectors. The method includes providing thearray such that each photodetector includes a photodiode having a p-njunction, where a wavelength at which a maximum spectral response of thephotodiode occurs is determined at least in part by a magnitude of areverse bias voltage applied across the p-njunction. Each of thephotodiodes includes a layer of semiconductor material that iscompositionally graded from wider bandgap material towards narrowerbandgap material in a direction away from the p-n junction, where thelayer confines substantially all (e.g., preferably more than about 95%,and more preferably more than about 99%) of a depletion region of thephotodiode. During operation of the array the method establishes foreach photodetector a predetermined magnitude of reverse bias voltage;and detects a signal generated from each photodetector that results fromincident electromagnetic radiation having wavelengths that correspond tothe maximum spectral response that is determined at least in part by themagnitude of the reverse bias voltage. The step of establishing mayestablish approximately the same magnitude of reverse bias voltage foreach photodetector of the array, or it may establish approximately thesame magnitude of reverse bias voltage for some of the photodetectors ofthe array while establishing at least one different magnitude of reversebias voltage for other photodetectors of the array, or the step ofestablishing may establish a different magnitude of reverse bias voltagefor each photodetector of the array. For a case where the array containsrows and columns of photodetectors, the step of establishing mayestablish a different magnitude of reverse bias voltage for individualones of rows or columns of the array. The step of establishing caninclude varying the magnitude of the reverse bias potential duringoperation of the array. For a case where the layer of semiconductormaterial is compositionally graded in a stepped fashion, increments ofreverse bias voltage can have a magnitude that is related to the steps.

An alternating current signal can be superimposed on the reverse DC biasvoltage and a synchronous detection technique used to detect photonscorresponding to a certain bandgap energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of these teachings are made more evidentin the following Detailed Description of the Preferred Embodiments, whenread in conjunction with the attached Drawing Figures, wherein:

FIG. 1 is a simplified cross-sectional view of a photodiode that isconstructed in accordance with the teachings of this invention;

FIG. 2 is an energy band diagram that corresponds to the photodiodeshown in FIG. 1;

FIGS. 3A, 3B and 3C are graphs depicting the composition profile,depletion depth vs. bias, and spectral cutoff vs. bias for thephotodiode shown in FIG. 1;

FIG. 4 is an energy band diagram of a multi-layer device with twoindependently tunable two color spectral responses;

FIG. 5 is a simplified cross-sectional view of a tunable detector inaccordance with the energy band diagram of FIG. 4;

FIG. 6 is an energy band diagram of a detector having a fixed cut-offLWIR region and a variable cut-on LWIR region that is a function of avariable cut-off of an MWIR responsive region;

FIG. 7 is an energy band diagram of a detector having a fixed LWIRbandpass and a variable cut-off MWIR bandpass;

FIGS. 8A, 8B and 8C are graphs depicting the band diagram, depthcomposition profile and depth doping profile for a non-linear or steppedprofile photodiode detector;

FIG. 9 illustrates a graph that depicts the spectral response vs.reverse bias (normalized to unity) for the stepped profile photodiodedetector;

FIGS. 10A, 10B and 10C depict an energy band diagram, depth compositionprofile and depth doping profile, respectively, for a device fabricatedusing the opposite polarity (p on n) of semiconductor material to theembodiments described by FIGS. 1-9;

FIG. 11A is a simplified top view of a staring type photodetector arraythat is biased so as to provide a uniform spectral response indicated bywavelength λ₁;

FIG. 11B is a simplified top view of a staring type photodetector arraythat is biased so as to provide a bandpass response indicated bywavelengths λ₁, λ₂ and λ₃;

FIG. 11C is a simplified top view of a multi-column scanning array whereeach column is biased differently to provide a graded spectra responseindicated by wavelengths λ₁, λ₂, λ₃, λ₄ and λ₅;

FIG. 12A is a graph that plots composition and doping profiles for anexemplary detector device; FIG. 12B illustrates the resulting energyband diagram and FIG. 12C shows the spectra response for two biasvoltages;

FIGS. 13A, 13B and 13C depict an energy band diagram, depth compositionprofile and depth doping profile for a device fabricated using a GroupIII-V material, specifically the material Al_(x)Ga_((1-x))As over thecomposition range for which it is a direct bandgap semiconductor; and

FIG. 14A is a simplified schematic diagram of a single photodetectorunit cell coupled to a readout and variable bias integrated circuit;

FIG. 14B is a timing diagram for the unit cell of FIG. 14A;

FIG. 15A is an enlarged top view of an exemplary 4×4 unit cell detectorarray; and FIG. 15B is an enlarged cross-sectional view of a portion ofthe array of FIG. 15A;

FIG. 16 is a block diagram, partly in schematic diagram form, of the 4×4array of FIGS. 15A and 15B; and

FIG. 17 is an exemplary ROIC unit cell timing diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein Short Wavelength Infrared (SWIR) radiation isconsidered to include a spectral region extending from approximately1000 nanometers (nm) to approximately 3000 nm. Medium WavelengthInfrared (MWIR) radiation is considered to include a spectral regionextending from approximately 3000 nm to approximately 8000 nm. LongWavelength Infrared (LWIR) radiation is considered to include a spectralregion extending from approximately 7000 nm to approximately 14000 nm.Very Long Wavelength Infrared (VLWIR) radiation is considered to includea spectral region extending from approximately 12000 nm to approximately30000 nm. Although the bands overlap to some extent, for the purposesdisclosed herein the overlap is not considered to be significant. Also,as employed herein a semiconductor material is considered to exhibitsignificant responsivity to a given spectral band if the semiconductormaterial exhibits a maximum or substantially maximum photosensitivity towavelengths within the given spectral band.

Referring to the photodetector 10 shown in cross-section in FIG. 1, andto the bandgap diagram of FIG. 2, in accordance with the teachings ofthis invention a highly doped wide bandgap p-type Hg_((1-x))Cd_(x)Telayer 14 is grown on a substantially transparent (at the wavelengths ofinterest) substrate 12. The layer 14 may be doped p-type using, forexample, Arsenic (As) having a concentration of about 10 ¹⁸ cm³. Thesubstrate 12 could be a suitable type of Group II-VI substrate, such asa CdTe substrate, or it could be a Silicon or other type of substratehaving appropriate accommodation and/or lattice matching layers grownthereon, if required. A lightly doped n-type Hg_((1-x))Cd_(x)Te layer 16is grown over the p-type layer 14, wherein the value of x is varied froma higher value (wider bandgap) to a lower value (narrower bandgap) in adirection away from the p-type layer 14, as shown in FIG. 3A. By sovarying the value of x during layer growth the n-type layer 16 iscompositionally graded through at least a portion of its thickness. Thelayer 16 may be doped n-type using, as an example, Indium (In) having aconcentration in a range of about 1×10¹⁴ cm³ to about 3×10¹⁴ cm³.Molecular Beam Epitaxy (MBE) is a presently preferred layer growthtechnique, although other layer growth techniques, such as Liquid PhaseEpitaxy (LPE) or Vapor Phase Epitaxy (VPE), could be used as well. Theresulting layers 14 and 16 form a p-n junction 15 where, because of thedifference in the doping concentrations of layers 14 and 16 and thecompositional grading profile of the n-type layer 16, the depletionregion 17 is predominantly and substantially exclusively located withinthe n-type layer 16. When the p-n junction is reverse biased it is thenobserved that the depletion region 17 is driven progressively deeperinto the n-type layer 16 as the magnitude of the reverse bias isincreased, as shown in FIG. 3B. This tends to overcome the electricfield caused by the composition gradient of the Hg_((1-x))Cd_(x)Te layer16, and photo-generated holes can be captured by the electric field inthe depletion region 17, instead of recombining outside of the depletionregion 17. Thus, increasing the magnitude of the reverse bias increasesthe spectral response cutoff wavelength of the p-n junction 15 of thephotodetector 10, when illuminated through the substrate 12, as shown inFIG. 3C.

Representative, but not limiting, layer thicknesses are about 2-3microns for p-type layer 14 and about 10-15 microns for the n-type layer16.

It is noted that the compositional grading profile of the layer 16 isopposite to that most often encountered (i.e., where the composition xwould be varied from a lower value (narrower bandgap) to a higher value(wider bandgap) in a direction away from the p-type layer 14 and towardsa surface of the device.)

General reference in this regard can be made to U.S. Pat. No.: 5,466,953by Rosbeck and Cockrum, ADenuded zone field effect photoconductivedetector@, where a compositionally graded HgCdTe radiation detector isconstructed to have a high purity denuded zone that is formed adjacentto a radiation absorbing region. The compositional grading results in aninternally generated electric field that is orthogonally disposed withrespect to an externally generated electric field applied betweencontacts. The internally generated electric field has the effect ofinjecting photogenerated minority charge carriers into the denuded zone,thereby reducing recombination with photogenerated majority chargecarriers and increasing carrier lifetime. The detector further includesa wider bandgap surface passivation region that functions to trap, or“getter”, impurities from the denuded zone and also to reduce surfacerecombination effects.

Reference can also be made to U.S. Pat. No.: 5,936,268 by Cockrum etal., AEpitaxial passivation of group II-VI infrared photodetectors@,where an array of photodiodes includes a radiation absorbing base layerof Hg_(1-x)Cd._(x)Te, where the value of x determines the responsivityof the array to either LWIR, MWIR or SWIR. The upper surface of thearray is provided with a passivation layer comprised of an epitaxiallayer of Group II-VI material which forms a heterostructure with theunderlying Group II-VI material and which has a wider bandgap than theunderlying Hg_(1-x)Cd._(x)Te, and which thereby repels both holes andelectrons from the diode junctions.

Further reference can also be had to U.S. Pat. No.: 5,880,510 by Cockrumet al., AGraded layer passivation of group II-VI infraredphotodetectors@, where a Group II-VI IR photodiode has a passivationlayer overlying at least exposed surfaces of a p-n diode junction. Thepassivation layer is a compositionally graded layer comprised of GroupII atoms diffused into a surface of the p-n diode junction. Thepassivation layer has a wider energy bandgap than the underlying diodematerial thereby repelling both holes and electrons away from thesurface of the diode and resulting in improved diode operatingcharacteristics. In the passivation layer the energy bandgap graduallydecreases in value as a function of depth from the surface until thebandgap energy equals that of the underlying bulk material.

In the instant photodiode 10 the compositional grading, from widerbandgap to narrower bandgap in a direction away from the p-n junction15, in combination with the difference in doping concentrations betweenlayers 14 and 16 (at least about one and preferably at least about twoor more orders of magnitude), causes the depletion region to reside orbe confined almost entirely within the n-type layer 16, and to grow orextend further into the layer 16 upon an increase in reverse bias, asopposed to growing into both the p-type layer 14 and the n-type layer16. Suitable dopant concentrations for the p-type layer 14 can be fromabout 10¹⁷ atoms/cm³ to about 10¹⁸ atoms/cm³, and suitable dopantconcentrations for the n-type layer 16 can be from about 3×10¹⁴atoms/cm³ to about 3×10¹⁵ atoms/cm³. Generally speaking, and by example,the dopant concentration of the p-type layer 14 is greater by about twoto three orders of magnitude than the dopant concentration of the n-typelayer 16.

It should be appreciated that this growth technique and the electricaloperation of the resulting photodetector 10 may be used with othervariable composition semiconductor materials to vary thephotosensitivity wavelength.

Furthermore, these teachings are not limited to the n-on-p photodetector10 embodiment shown in FIG. 1. For example, FIGS. 10A, 10B and 10Cdepict an energy band diagram, depth composition profile and depthdoping profile, respectively, for a device fabricated using the oppositepolarity (p-on-n) of Group II-VI semiconductor material to theembodiment described by FIGS. 1, 2 and 3A-3C, as well as those describedbelow in reference to FIGS. 4-9. In this case the n-type material ismore heavily doped than the p-type material, and the compositionalprofile of the p-type material is graded.

In addition, these teachings may be applied to other than Group II-VIdetectors of IR radiation. For example, these teachings may be appliedas well to photodetectors constructed using Group III-V materials.Examples of III-V semiconductor materials that are suitable forsupporting the bias tuneable bandgap in accordance with these teachingsinclude, but need not be limited to, the following:

-   -   Al_(x)In_((1-x))P    -   Ga_(x)In_((1-x))P    -   InP_(x)As_((1-x))    -   Al_(x)Ga_((1-x))As    -   Ga_(x)In_((1-x))As    -   InAs_(x)Sb_((1-x))    -   Al_(x)In_((1-x))As    -   Ga_(x)In_((1-x))Sb    -   InGaN    -   Al_(x)Ga_((1-x))    -   SbGaP_(x)As_((1-x))    -   Ga_(x)In_((1-x))As_(y)P_((1-y))    -   Al_(x)In_((1-x))Sb    -   GaAs_(x)Sb_((1-x))

As but one example, FIGS. 13A, 13B and 13C illustrate an energy banddiagram, depth composition profile and depth doping profile for aphotodetector device fabricated using the material Al_(x)Ga_((1-x))Asover the composition range for which it is a direct bandgapsemiconductor. This device as well features the significant dopingconcentration difference across the p-n junction and the compositionallygraded layer that cooperate to support the confinement and growth of thedepletion region 17 within the more lightly doped layer under increasingreverse bias conditions.

By superimposing a smaller AC voltage on the DC bias voltage, theresulting AC diode current can be treated as a spectral bandpass signal.The magnitude of the AC voltage determines the spectral width of thebandpass. By sweeping the DC bias voltage, the spectral bandpass is thenswept across a range of the IR spectrum. By using phase sensitive ACdetection, such as synchronous detection techniques that are phaselocked to the phase of the AC bias voltage, the signal-to-noise ratio(SNR) of the detector 10 may be significantly improved.

In this case a p-n junction has a graded composition in a low dopedlayer in which the depletion region can be pushed into increasinglynarrower bandgap material as the reverse bias is increased. Synchronousdetection of the small AC current resulting from the small AC bias thatis superimposed on the swept DC reverse bias is used to obtain avariable spectral response to infrared photon stimulation.

In one non-limiting example a −1 volt DC bias has a 20 mV RMS, 1 kHz ACsignal superimposed thereon. A synchronous detector is used to detectonly the AC component, where the synchronous detector is phase andfrequency locked to the AC component of the bias signal. The DC biasvoltage can be held at a constant value, or it can be swept over a rangeof voltage values. In this aspect of the invention one essentiallymeasures only those IR photons having an energy that corresponds to abandgap energy that is related to the AC bias component and the presentvalue of the DC bias.

-   -   A multilayer device with two independently tunable two color        spectral responses may also be fabricated. For example, FIG. 4        is an energy band diagram of a multi-layer device with two        independently tunable two color spectral responses (e.g., LWIR        and MWIR), and FIG. 5 is a simplified cross-sectional view of a        tunable detector 30 in accordance with the energy band diagram        of FIG. 4. In FIG. 5 the substrate 12 and p+ layer 14 may be as        described above for the photodetector 10 of FIG. 1, as may the        n− layer 16. Representative, but not limiting, layer thicknesses        are about 2-3 microns for p+ layer 14 and about 10-15 microns        for the n− layer 16.

Over the n− layer 16 is then grown a p-type barrier layer 18, and thenanother compositionally graded n-type layer 20. Representative, but notlimiting, layer thicknesses are about 2-3 microns for the p barrierlayer 18 and about 10-15 microns for the n− layer 20. The resultinglayered structure is photolithographically processed into pixel or unitcell mesa structures each comprised of a first, primary mesa 30A and asecondary mesa 30B. Suitable electrical contact pads and interconnects,such as Indium bumps 22A and 22B are then added. Indium bump 22Aprovides electrical contact for a readout and variable biasingintegrated circuit (ROVBIC) 32. Metallization 24 provides anelectrically conductive path from Indium bump 22B to the p barrier layer18 contained with the primary mesa 30A wherein incident MWIR and LWIRradiation is detected. In this embodiment, as shown in FIG. 4, thedetection wavelength of both the MWIR and the LWIR radiation is biastuneable.

FIG. 14A is a simplified schematic diagram of a single photodetectorunit cell 10, as in FIG. 1, coupled to the ROVBIC 32. A Detector CommonBias 32C attaches to a ground ring of the detector array (shown moreclearly in FIG. 15A), and sets the detector common voltage to a levelthat is optimum for a capacitance transimpedance amplifier (CTIA). ADetector Bias DAC (digital to analog converter) 32A sets the biasvoltage on the detector diode 10 through the CTIA 32B, when anIntegration Switch (S1) is closed. This bias voltage sets the spectralresponse of the detector array. The above-mentioned AC component of thebias voltage can be added by the DAC 32A by varying the input digitalvalue at a rate and over a range of values to provide the desired ACmodulation of the DC bias. The AC component would then also be providedto a synchronous detector enabling phase and frequency lock to the ACbias component. The DC bias voltage value can be varied or swept aswell, as was also mentioned previously.

In operation, and referring also to FIG. 14B (which shows twointegration periods), the Integration Switch S1 and a Reset Switch (S2)are closed, and a Row Switch (S3) is open. This puts the CTIA 32B into aunity gain mode, and the voltage produced by the Detector Bias DAC 32Aappears across the Detector Diode 10. Then the Reset Switch S2 isreleased, and electrical charge from the Detector Diode 10 flows into anintegration capacitor (Cint), since the charge cannot flow into theinverting input of the CTIA 32B. As the charge flows into Cint thevoltage across it increases. However, the CTIA 32B operates to maintainits + and B inputs at the same voltage, which in this case is thevoltage output by the Detector Bias DAC 32A. The result is that ascharge continues to flow into Cint, the voltage on the output node ofthe CTIA 32B increases negatively. When the integration time is over,the Integration Switch S1 is opened. At some time later the Row SwitchS3 is closed, and the output voltage of the CTIA 32B is output from theROVBIC 32. The output waveform in FIG. 14B shows twice the amount ofvoltage change between 20 ms and 29 ms as between 4 ms and 13 ms. Thiswould occur if the detector photo current were to double between the twointegration periods.

A photodetector device with a fixed cutoff LWIR and a variable LWIRAcuton@, depending on the variable cutoff of the MWIR, is shown in FIG.6. Only one indium bump 22 per pixel is required for this embodiment, asopposed to the use of two Indium bumps 22A and 22B in FIG. 5. Inessence, in this embodiment the secondary mesa 30B of FIG. 5 iseliminated.

FIG. 7 is an energy band diagram of a detector having a fixed LWIRbandpass and a variable cut-off MWIR bandpass. In this embodiment then-barrier prevents carriers generated to the right of it from reachingthe LWIR junction when it is accessed. Only one Indium bump 22 per pixelis required. Applying a negative bias to the left terminal (the terminalopposite the entry of the light) with respect to the right terminalreverse biases the left p-n junction 15A and activates the LWIR mode.Applying a positive bias activates the right p-n junction 15B, withtuneable cutoff wavelength by varying the positive bias voltage.

Any of the preceding structures that implement the compositionalgradient may also employ the non-linear or step profile shown in FIGS.8A-8C, where FIGS. 8A, 8B and 8C show the band diagram, the depthcomposition profile and the depth doping profile, respectively, for thestepped profile photodiode detector, as opposed to the smoothly gradedor linear composition profile photodiode detector. FIG. 9 shows a graphthat depicts the spectral response vs. reverse bias (normalized tounity) for the stepped profile photodiode detector. Each step in thecomposition of the n-type layer 16 is bounded on the left edge by aminority carrier reflector, to improve quantum efficiency. The resultingphotodiode exhibits a stepped spectral cutoff response, and smallvariations of bias voltage does not affect the response. By varying thebias voltage between two steps, a spectral bandpass response resultsfrom the difference signal detected at each of the two steps. Thisembodiment gives the same spectral response across an array ofdetectors, even if the bias voltage varies a small amount betweendetectors, and/or if the depletion depth varies because of dopingvariations. A detector array fabricated with this step approach may bemore producible, and may require less scene correction. In addition, thesteps may be adjusted to make the detector sensitive to specificspectral bands or spectral cutoffs.

While the stepped structure illustrated in FIGS. 8A-8C and 9 may appearat first glance to be superficially similar to that of Brüggermann etal. (J. Appl. Phys. 81, 7666 (1997)), Brüggermann et al. did not rely onthe barrier height of the isotype heterojunction to provide separationof photocarriers according to the wavelength of the absorbed light.

A two dimensional focal plane array 40 may be fabricated from any of thepreviously described structures. The array 40 may be biased uniformly soit responds uniformly in spectral response, as in FIG. 11A, or thespectral response may be electrically graded across the array 40, as inFIG. 11 B. The graded response may be useful for scanning applications,as in FIG. 11C.

More specifically, FIG. 11A is a simplified top view of a staring typephotodetector array 40 that is biased so as to provide a uniformspectral response indicated by wavelength λ₁.

An AC component may be applied to the detector substrate 12 to achieve abandpass response. For example, FIG. 11B is a simplified top view of thestaring type photodetector array 40 that is selectively biased so as toprovide a bandpass response indicated by wavelengths λ₁, λ₂ and λ₃. FIG.11C is a simplified top view of a multi-column scanning type of array40, where each column is biased differently, indicated by voltages V₁,V₂, V₃, V₄ and V₅, to provide a graded spectral response indicated bywavelengths λ_(1, λ) ₂, λ₃, λ₄ and λ₅, respectively. In this embodimentthe scanning direction of the incident light is perpendicular to thecolumns, and each column is sensitive to a different range ofwavelengths arriving from the scene being viewed. In other embodimentseach row may be biased differently.

In general, in various embodiments in accordance with this inventioneach of n photodetectors of the array 40 could be reverse biaseddifferently so as provide a maximum spectral response to individual onesof λ₁ through λ_(n) different wavelengths. In another embodiment some ofthe photodetectors of the array could have a fixed spectral response,i.e., one that is not bias voltage tuneable, while others of thephotodetectors of the array could have the adjustable spectral response,i.e., one that is bias voltage tuneable. In any of these embodiments thereverse bias voltage could be varied and then set prior to operation, orthe reverse bias voltage may be varied during operation (for example atthe frame rate or at a multiple of the frame rate).

Reference is now made to FIG. 12A for showing a graph that plotscomposition and doping profiles for an exemplary photodetectorconstructed in accordance with these teachings, while FIG. 12Billustrates the resulting energy band diagram and FIG. 12C shows thespectral response for two different (and exemplary) bias voltages (i.e.,V_(DET)=0V then λ_(c)=2.23 μm and V_(DET)=5V then λ_(c)=3.98 μm).

An exemplary 4×4 detector array 100 may be fabricated as shown in FIGS.15A and 15B. The 4×4 array of pixels are surrounded by a ring of groundcontacts 102 to the detector common p+ layer 14. The detector array 100may be connected to the readout integrated circuit (ROIC), or ROVBIC 32in this case, with the indium bumps 22A that mate with correspondingbumps 104A. The ground contacts 102 mate with corresponding bumps 104B.

An electrical diagram for the ROVBIC 32 is shown in FIG. 16. Note thateach of the unit cell schematic diagrams corresponds to the onediscussed above with respect to FIG. 14A. Each detector diode cathode 16connects to a unit cell input circuit, which in this case is the CTIA32B. The detector anodes 14 all form the p+ common connection, whichattaches to the ROVBIC 32 by the ground contact ring 102 shown in FIGS.15A and 15B.

Also shown in FIG. 16 is a clock generator block 34 that generates theIntegrate and Reset timing signals for switches S1 and S2, respectively,and a row shift register block 36 that generates the Row switch S3timing signals sequentially in a row-by-row manner. The unit celloutputs are input to one of four column amplifiers 38 that in turn feeda chip output amplifier 40 via a column shift register 42. A detectorbias generator block 44 includes the Detector Bias DAC 32A and alsogenerates the Detector Bias Common 32C, both shown in FIG. 14A.

A Digital block 46 receives digital signals (clock and data) from anexternal controller or data processor (not shown), and converts thedigital signals into column readout signals that are applied to thecolumn shift register for controlling the outputting of unit cellsignals. Another output of the Digital block 46 is applied to theDetector Bias Generator block 44 for setting at least the level of theoutput of the Detector Bias DAC 32A.

FIG. 17 shows the timing waveforms for the 4×4 array 100 of FIGS. 15Aand 15B, and illustrates the operation of the circuitry shown in FIG.16. The output of the array 100 (the bottom trace) results in thisexample from imaging the red and blue scenes shown below the timingdiagram. The blue scene is assumed to cover every other pixel, while thered scene is assumed to cover the center two pixel columns of the array100. The time from the first high pulse on Data to the next high pulseis referred to as a frame; and the exemplary frame period in FIG. 17 isthus 11 ms. The Data may be used to program the Detector Bias DAC 32A,among other necessary functions, as described above. In thisillustration the Detector Bias DAC 32A is programmed to provide detectorsensitivity to a relatively short wavelength scene in the first frame.In the second frame, the Detector Bias DAC 32A is programmed to providelonger wavelength sensitivity (−0.5 volts versus −1.0 volts,respectively). The Reset and Integrate timing signals, and the Biasvoltage, affect all of the detectors 10 on the array 100 simultaneously.After reset and integration, all sixteen unit cell voltages aremultiplexed to the single output amplifier 40, producing a video streamof analog voltages. This video stream may then be converted back into aviewable image, and/or it may be processed using any of a number ofknown types of algorithms such as those that enhance the image, removeartifacts, perform uniformity corrections, and so forth.

Note in this example that the Detector Bias DAC 32A can be programmedduring operation, e.g., on a frame-by-frame basis, to provide thedesired sensitivity to different wavelength bands.

Note as well that the output of the Detector Bias DAC 32A is shown asbeing applied in common to all of the unit cells. It is, however, withinthe scope of these teachings to provide two or more Detector Bias DACs32A, and suitable selection and multiplexing circuitry, for applyingdifferent Detector Bias signals to different photodetectors 10. In thismanner, and by example, during a single frame certain ones of thephotodetectors 10 are made responsive to a first wavelength band andcertain others of the photodetectors 10 are made responsive to a secondwavelength band that may or may not completely or partially overlap thefirst wavelength band.

In addition, and as was discussed above, a smaller AC signal can besuperimposed on the DC bias voltage, where the magnitude of the ACsignal determines the spectral width of the bandpass. By sweeping the DCbias voltage the spectral bandpass is swept across a range of the IRspectrum. By using a phase sensitive AC detection technique, such as asynchronous detection technique that is phase locked to the phase of thesuperimposed AC signal, the SNR of the detector 10 can be significantlyimproved.

While described in the context of exemplary semiconductor materials,dopants, dopant concentrations, layer thicknesses, compositionalprofiles, wavelengths, bias voltages, circuit embodiments, waveformlevels and times, these are intended to be viewed in a non-limiting andexemplary sense, and are not intended to be construed as limiting thescope or practice of the teachings in accordance with this invention.

1. A photodetector, comprising: a substrate having a surface; a first layer comprised of semiconductor material that is disposed above said surface, said first layer comprising a first dopant at a first concentration for having a first type of electrical conductivity; and a second layer comprised of semiconductor material overlying said first layer, said second layer comprising a second dopant at a second concentration for having a second type of electrical conductivity and forming a first p-n junction with said first layer, said second layer being compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap in a direction away from said p-n junction, where said first concentration is at least an order of magnitude greater than said second concentration, and where when said first p-n junction is reverse biased, a depletion region exists substantially only within said second layer.
 2. A photodetector as in claim 1, where said semiconductor material is comprised of Group II-VI material.
 3. A photodetector as in claim 1, where said semiconductor material is comprised of Group III-V material.
 4. A photodetector as in claim 1, where said first type of electrical conductivity is p-type, and where said second type of electrical conductivity is n-type.
 5. A photodetector as in claim 1, where said first type of electrical conductivity is n-type, and where said second type of electrical conductivity is p-type.
 6. A photodetector as in claim 1, where said second layer is compositionally graded through at least a portion of a thickness thereof in a substantially linear fashion.
 7. A photodetector as in claim 1, where said second layer is compositionally graded through at least a portion of a thickness thereof in a substantially step-wise fashion.
 8. A photodetector as in claim 1, and comprising at least one electrical contact for coupling said second layer to a source of bias voltage.
 9. A photodetector as in claim 1, and further comprising at least one electrical contact for coupling said second layer to a source of variable bias voltage for reverse biasing said p-n junction, where as a magnitude of the bias voltage is changed a wavelength of electromagnetic radiation to which said photodetector is responsive is changed.
 10. A photodetector as in claim 1, and further comprising: a third layer comprised of semiconductor material that is disposed above said second layer, said third layer comprising a third dopant at a third concentration for having the first type of electrical conductivity; and a fourth layer comprised of semiconductor material overlying said third layer, said fourth layer comprising a fourth dopant at a fourth concentration for having the second type of electrical conductivity and forming a second p-n junction with said third layer, said fourth layer being compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap semiconductor material in a direction away from said second p-n junction, where said third concentration is at least an order of magnitude greater than said fourth concentration, and where when said second p-n junction is reverse biased, a depletion region exists substantially only within said fourth layer of semiconductor material.
 11. A photodetector, comprising: a first layer comprised of semiconductor material, said first layer comprising a first dopant at a first concentration for having a first type of electrical conductivity; a second layer comprised of semiconductor material overlying said first layer, said second layer comprising a second dopant at a second concentration for having a second type of electrical conductivity and forming a first p-n junction with said first layer, said second layer being compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap in a direction away from said p-n junction; a first electrical contact for coupling said second layer to a first bias voltage for reverse biasing said first p-n junction; a third layer comprised of semiconductor material that is disposed above said second layer, said third layer comprising a third dopant at a third concentration for having the first type of electrical conductivity; a fourth layer comprised of semiconductor material overlying said third layer, said fourth layer comprising a fourth dopant at a fourth concentration for having the second type of electrical conductivity and forming a second p-n junction with said third layer, said fourth layer being compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap semiconductor material in a direction away from said second p-n junction; and a second electrical contact for coupling said fourth layer of semiconductor material to a second bias voltage for reverse biasing said second p-n junction; where said first concentration is at least an order of magnitude greater than said second concentration and said third concentration is at least an order of magnitude greater than said fourth concentration, and where when said first p-n junction is reverse biased, a depletion region exists substantially only within said second layer of semiconductor material, and when said second p-njunction is reverse biased, a depletion region exists substantially only within said fourth layer of semiconductor material.
 12. A photodetector as in claim 11, where said first bias voltage is a variable bias voltage for varying a wavelength of electromagnetic radiation to which said photodetector is responsive.
 13. A photodetector as in claim 11, where said second bias voltage is a variable bias voltage for varying a wavelength of electromagnetic radiation to which said photodetector is responsive.
 14. A photodetector as in claim 11, where said first bias voltage differs in magnitude from said second bias voltage.
 15. An array of infrared radiation responsive photodetectors, each photodetector comprising a photodiode having a p-n junction, wherein a wavelength at which a maximum spectral response of said photodiode occurs is determined at least in part by a magnitude of a reverse bias voltage applied to said p-n junction, wherein each of said photodiodes comprises a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from said p-n junction, said layer confining substantially all of a depletion region of the photodiode.
 16. An array of infrared radiation responsive photodetectors, each photodetector comprising a first photodiode in series with a second photodiode, each photodiode having an associated p-n junction, wherein a wavelength at which a maximum spectral response of a first one of said first and second photodiodes occurs is determined at least in part by a magnitude of a reverse bias voltage applied to said associated p-n junction, said first one of said first and second photodiodes comprising a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from said p-n junction, said layer confining substantially all of a depletion region of the photodiode, and wherein a wavelength range in which a maximum spectral response of the other one of said first and second photodiodes occurs is determined at least in part by the wavelength at which a maximum spectral response of a first one of said first and second photodiodes occurs.
 17. An array of infrared radiation responsive photodetectors, each photodetector comprising a first photodiode in series with a second photodiode, each photodiode having an associated p-n junction, wherein a wavelength at which a maximum spectral response of a first one of said first and second photodiodes occurs is determined at least in part by a magnitude of a reverse bias voltage applied to said associated p-n junction, said first one of said first and second photodiodes comprising a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from said p-n junction, said layer confining substantially all of a depletion region of the photodiode, and wherein a wavelength range at which a maximum spectral response of a second one of said first and second photodiodes occurs is determined at least in part by a magnitude of a reverse bias voltage applied to said associated p-n junction, said second one of said first and second photodiodes also comprising a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from said associated p-n junction, said layer confining substantially all of a depletion region of the second one of said first and second photodiodes. 18-31. Cancelled
 32. A photodetector as in claim 1, where said second layer is compositionally graded through at least a portion of the thickness thereof in a step-wise fashion with adjacent steps separated by a minority carrier reflector.
 33. An array of infrared radiation responsive photodetectors, each of said photodetectors of the array of infrared radiation responsive photodetectors comprising a photodiode that comprises a p-n junction, where a wavelength at which a maximum spectral response of said photodiode occurs is determined at least in part by a magnitude of a bias voltage applied to said p-n junction by an associated readout circuit, said associated readout circuit comprising a part of an array of readout circuits, said readout circuit comprising circuitry for reading out electrical signals generated during an integration period by said photodetector, said array of readout circuits further comprising at least one variable source of bias voltage having an output coupled to said array of infrared radiation responsive photodetectors and controlled so as to set the wavelength at which the maximum spectral response occurs, where an individual one of said array of radiation responsive bias tunable photodetectors is coupled to a first input of an integration amplifier of an associated one of said readout circuits and where said at least one variable source of bias potential is coupled to a second input of said integration amplifier and across said photodetector for a first portion of an integration period by operating said integration amplifier in a unity gain mode of operation by closing a switch coupled across an integration capacitance that is coupled between said first input and an output of said integration amplifier, and where said switch is opened during a second portion of said integration period for enabling charge from said photodetector to be integrated on said integration capacitance.
 34. An array of infrared radiation responsive photodetectors as in claim 33, where each of said photodiodes comprises a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from said p-n junction, said layer confining substantially all of a depletion region of the photodiode. 